CN111788731B - Fuel cell system and method for operating fuel cell system - Google Patents

Fuel cell system and method for operating fuel cell system Download PDF

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Publication number
CN111788731B
CN111788731B CN201980017321.3A CN201980017321A CN111788731B CN 111788731 B CN111788731 B CN 111788731B CN 201980017321 A CN201980017321 A CN 201980017321A CN 111788731 B CN111788731 B CN 111788731B
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gas
steam
anode
fuel
fuel cell
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CN111788731A (en
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神家规寿
越后满秋
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Osaka Gas Co Ltd
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Osaka Gas Co Ltd
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    • C01B3/02—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen
    • C01B3/32—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air
    • C01B3/34—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents
    • C01B3/38—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents using catalysts
    • C01B3/384—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents using catalysts the catalyst being continuously externally heated
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    • H01M8/0612—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants from carbon-containing material
    • H01M8/0618—Reforming processes, e.g. autothermal, partial oxidation or steam reforming
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    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B90/00—Enabling technologies or technologies with a potential or indirect contribution to GHG emissions mitigation
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Abstract

The fuel cell SOFC that generates electricity by supplying the reformed gas obtained by steam reforming to the fuel electrode is provided with a fuel cell system capable of further improving the electricity generation efficiency as compared with the current state. The fuel cell system includes: a steam reformer (1) for reforming a hydrocarbon fuel by a steam reforming reaction; a fuel cell (101) that functions by introducing the reformed gas into a fuel electrode (s 2); and an anode off-gas circulation path (8) for cooling the anode off-gas (g 3 a) and removing condensed water, introducing the cooled water into the steam reformer (1), and controlling the condensing temperature in the condensing means (13) by means of a control means (C) for controlling the partial pressure of the steam of the anode off-gas (g 3 a) circulating in the steam reformer (1), so that S/C regulation corresponds to high-efficiency power generation.

Description

Fuel cell system and method for operating fuel cell system
Technical Field
The present invention relates to a fuel cell system and an operation method thereof, the fuel cell system comprising: a steam reformer for reforming a hydrocarbon fuel through a steam reforming reaction; a fuel cell that functions by introducing the reformed gas obtained by the steam reformer to a fuel electrode; and an anode off-gas circulation path for cooling anode off-gas (anode off gas) discharged from the fuel electrode, removing condensed water, and introducing the cooled water into the steam reformer.
Background
As such a fuel cell system, the technologies disclosed in patent document 1 and patent document 2 are exemplified.
The technology disclosed in patent document 1 has an object of: the hydrogen-containing gas or the like can be recycled regardless of the location within the system, without the need for additional dedicated moisture removal structures for the recycled gas. Accordingly, a fuel cell system is proposed, which includes: a hydrogen production apparatus 1 including a desulfurization unit 2, a reforming unit 6 (corresponding to a steam reformer of the present invention), and a selective oxidation reaction unit 8; and a fuel cell that generates electricity using the reformed gas generated by the hydrogen production apparatus 1.
Further, in paragraph [0039], it is disclosed that: the fuel cell system shown in fig. 1 and 2 includes a functional unit as a moisture removing unit 44 that removes moisture contained in the recirculated anode off-gas. The moisture removing unit 44 includes: a heat exchanger 44A provided between the cell stack 20 and the branching portion BP3 for cooling the anode off-gas; and a drain recoverer 44B that recovers the condensed water. In order to remove moisture from the anode off-gas before being supplied to the combustor 10, the moisture removing portion 44 has been conventionally incorporated. The anode off-gas from which moisture has been removed in the moisture removing portion 44 is mixed with the raw fuel via the recirculation line RL 3.
The technology disclosed in patent document 2 has an object of: provided is a solid oxide fuel cell system wherein carbon is not likely to precipitate in the reformer and the reformer (corresponding to the steam reformer of the present invention) is not likely to excessively heat up. Accordingly, the solid oxide fuel cell system includes: a reformer; a solid oxide fuel cell (corresponding to the fuel cell of the present invention); an anode exhaust gas recirculation path; an anode off-gas supplier that adjusts a supply amount of anode off-gas supplied to the reformer; an anode off-gas radiator that radiates heat from the anode off-gas to generate condensed water; a reformer temperature detector; and a controller that controls at least any one of the reforming air supply device, the raw material supply device, and the anode off-gas supply device according to the temperature of the reformer, thereby adjusting at least any one of the reforming air supply amount, the raw material supply amount, and the anode off-gas supply amount.
The function of the controller 19 is described in paragraph [0052 ].
The controller 19 controls at least any one of the reforming air supplier 10, the raw material supplier 12, and the anode off-gas supplier 24 based on the temperature of the reformer 14 detected by the reformer temperature detector 28, thereby adjusting at least any one of the reforming air supply amount to the reformer 14, the raw material supply amount to the reformer 14, and the anode off-gas supply amount to the reformer 14.
Therefore, the technique disclosed in patent document 2 is intended to maintain the temperature of the reformer 14 in a good state.
Prior art literature
Patent literature
Patent document 1: japanese patent application laid-open No. 2011-210634;
patent document 2: japanese patent application laid-open No. 2014-089919.
Disclosure of Invention
Problems to be solved by the invention
However, in order to increase the efficiency of the fuel cell, it is necessary to consume fuel by generating electricity as much as possible. In this case, if the consumption rate is high, the fuel partial pressure at the outlet side of the fuel electrode is lowered to cause a fuel deficiency, which in turn lowers the performance of the fuel cell, and may cause fatal damage to the cell due to uneven cell reaction distribution.
On the other hand, in a fuel cell (for example, an oxygen ion permeable electrolyte fuel cell (a solid oxide fuel cell SOFC of this type)) that generates electricity by a steam reforming reaction using hydrocarbons as a raw fuel and using the reformed gas (including hydrogen and carbon monoxide) as a direct fuel of the fuel cell, the reaction rate of the steam reforming reaction is increased, and the reformed gas is generated as much as possible and used for electricity generation as much as possible, which contributes to an improvement in the electricity generation efficiency. Therefore, it is necessary to increase both the reaction rate of the steam reforming and the utilization rate of the fuel gas (reformed gas) as much as possible.
However, in the present situation, if a large amount of power is generated in the fuel cell, the amount of water and carbon dioxide gas generated by the power generation reaction also increases accordingly. As a result, the water vapor partial pressure at the fuel electrode outlet of the fuel cell increases, resulting in a direct drop in the fuel partial pressure of the fuel. In addition, if anode off-gas higher than a prescribed water vapor partial pressure or an excessive amount is supplied to the water vapor reformer by recirculation, the fuel partial pressure of the fuel is directly lowered. When the fuel partial pressure becomes too low, the amount of diffusion supply of the fuel gas to the fuel electrode decreases, and therefore the amount of power generation reaction per unit area of the fuel electrode decreases, and the amount of heat generation of the fuel cell by the power generation reaction decreases. This phenomenon causes a fatal failure such as deformation or cracking of the fuel electrode due to non-uniformity of temperature distribution caused by the expansion of the temperature difference between the inlet and outlet of the fuel electrode as a result of the expansion of the temperature difference between the inlet and outlet of the fuel cell.
As one of methods for avoiding this problem, a method of reducing the steam supplied to the steam reforming catalyst from the raw fuel supply side together with the raw fuel to increase the fuel partial pressure is considered. However, the equilibrium composition in the steam reforming reaction tends to be a direction of reducing the fuel of hydrogen or carbon monoxide supplied to the fuel electrode, and the partial pressure of the fuel gas supplied to the fuel cell is lowered, which is not a fundamental solution. In addition, a method of increasing the reaction rate by further increasing the steam reforming temperature is also considered, but in reality the heat resistant temperature of the steam reforming catalyst is limited, so that the long-term durability of the catalyst is reduced by increasing the steam reforming temperature.
As described above, in the present situation, if considering the fuel gas partial pressure at the fuel electrode outlet that can be tolerated by the fuel electrode of the fuel cell, if the ratio of the amount of steam to the amount of carbon supplied to the steam reformer is set to, for example, S/C ratio=2.6, the fuel utilization (the amount of fuel consumed in the fuel cell/the amount of fuel supplied to the fuel cell) is set to 80% under the condition that the steam reforming equilibrium temperature is about 670 ℃, the cell voltage is set to 0.8V, and the dc conversion efficiency and the auxiliary equipment efficiency are set to 0.95, respectively, the output efficiency of ac power is generally limited to 52 to 55%.
As described above, the main object of the present invention is to: as for a fuel cell that supplies reformed gas obtained by steam reforming to a fuel electrode to generate electricity, a fuel cell system capable of further improving the power generation efficiency as compared with the current state of the art is provided, and a method of operating such a fuel cell system is provided.
Means for solving the problems
The 1 st feature of the present invention for solving the above problems is that:
a fuel cell system is provided with:
a steam reformer for reforming a hydrocarbon fuel through a steam reforming reaction;
a fuel cell that functions by introducing the reformed gas obtained by the steam reformer to a fuel electrode; and
An anode off-gas circulation path for cooling the anode off-gas discharged from the fuel electrode and removing condensed water, and introducing the cooled anode off-gas into the steam reformer,
and a control means (means) for controlling the partial pressure of the water vapor of the anode off-gas circulated in the steam reformer by adjusting the amount of the condensed water removed by the anode off-gas circulation path.
In the fuel cell system of this configuration, the hydrocarbon fuel is supplied to the steam reformer from the raw fuel supply side, and the steam is supplied to the steam reformer via the anode off-gas circulation path.
When both are supplied, the control means functions to adjust the partial pressure of water vapor of the anode off-gas circulated from the anode off-gas circulation path. By performing such adjustment, even if the power generation amount of the fuel cell is increased and the water generation amount in the fuel cell is increased, the water vapor partial pressure of the anode off-gas is reduced by the condensation operation, so that sufficient water vapor required for the water vapor reforming can be supplied, and the amount of fuel gas supplied to the fuel cell can be increased through the water vapor reforming. As a result, the amount of hydrocarbon fuel, which is a raw material that can be input to the steam reformer, can be appropriately adjusted, with the result that the power generation efficiency is improved.
The above is the main action and effects obtained by adopting the 1 st feature configuration of the present invention, and more specifically, the following actions and effects can be exhibited.
Direct action/effect by controlling partial pressure of water vapour
1. The amount of fuel gas supplied to the fuel cell can be increased, so that even if the amount of power generation of the fuel cell is the same (as in the case where the anode off-gas circulation is not performed), the fuel gas concentration at the fuel electrode outlet of the fuel cell can be increased. As a result, the previously described problem of excessive drop in fuel gas partial pressure at the fuel electrode outlet can be avoided.
In contrast, even when the fuel consumed by the power generation reaction is increased in the fuel cell (when the power generation amount is increased), the necessary fuel gas partial pressure at the fuel electrode outlet side can be ensured, and the fuel gas partial pressure can be ensured to the extent of the conventional level at the fuel electrode outlet, without causing any problem.
2. Since the partial pressure of the fuel gas at the inlet and the outlet of the fuel electrode is close to each other, the current density in the fuel cell is uniformized, so that local overheating due to the power generation resistor is suppressed, and the long-term reliability of the fuel cell is improved.
When one example of a specific study by the inventors of the present invention is shown, the fuel gas flow rate to the fuel cell is increased by circulating the anode off-gas, so that the concentration difference between the fuel gas at the inlet and the fuel gas at the outlet of the fuel electrode is: the case of the prior art (the fuel cell system S2 shown in fig. 2) is about 54%, whereas the case of the present invention (the fuel cell system S1 shown in fig. 1) is reduced to about 34%. As described above, although the temperature distribution increases due to the uneven distribution of the power generation reaction in the fuel gas concentration difference inside the cell, the thermal deformation of the fuel cell is alleviated by the alleviation of the concentration difference in the present invention, and the long-term durability is improved.
That is, by circulating the anode off-gas, the partial pressure of which has been appropriately controlled, in the steam reformer, the power generation efficiency can be improved while ensuring the amount of fuel gas that can be supplied to the fuel cell and the amount of steam required for reforming thereof.
On the other hand, the following functions and effects are obtained by using the steam held in the anode off-gas for reforming. Compared to the existing solid oxide fuel cell system shown in fig. 2.
1. In the normal operation in which the generated power is supplied to the outside in accordance with the power load, the steam supplied to the steam reforming catalyst may be only the steam in the anode off-gas, and therefore, does not include substances that adversely affect the steam reforming catalyst, such as iron oxide, silica, or sulfides contained in the combustion off-gas or the cathode off-gas (cathode off-gas). As a result, poisoning of the reformer catalyst, contamination or clogging of the heat exchanger, or the like is suppressed, and the long-term reliability of the fuel cell process is improved.
2. In the fuel cell system according to the present invention, the power generation state in which the condensed water is constantly discharged in the normal operation can be realized, and the above-described constant discharge amount can ensure a molar number of about 15% with respect to the amount of the raw fuel, so that the concentration of the fuel system contaminant component in the process into the condensed water does not occur. Thus, the possibility of fouling of the catalyst or the apparatus is eliminated, and the long-term reliability is improved.
3. Since water contained in the combustion exhaust gas system and the cathode exhaust gas system is not utilized as water required for steam reforming, high-level water treatment for removing silica or the like can be simplified or omitted.
Further, since the condensed water can be constantly discharged to the outside of the system and the contaminated components eluted from the reactor, piping, valve and the like are not concentrated, the purity of water can be maintained for a long period of time even without a water purifier.
By adopting the present characteristic configuration, the operation method of the fuel cell system is as follows:
the operation method of the fuel cell system includes:
a steam reformer for reforming a hydrocarbon fuel through a steam reforming reaction;
a fuel cell that functions by introducing the reformed gas obtained by the steam reformer to a fuel electrode; and
an anode off-gas circulation path for cooling the anode off-gas discharged from the fuel electrode, removing condensed water, and introducing the cooled water into the steam reformer,
and, the amount of condensed water removed from the anode off-gas circulation path is adjusted to adjust the water vapor partial pressure of the anode off-gas circulated in the water vapor reformer.
The 2 nd feature of the present invention is constituted by:
A condensing means for removing condensed water on the discharge side, suction side, or both sides of the circulating means for circulating the anode off-gas to the steam reformer,
the control means adjusts the circulation amount of the circulation means and the condensation temperature in the condensation means to adjust the amount of steam circulated in the steam reformer.
By virtue of the constitution of the present feature,
the condensing means is provided on the discharge side or suction side or both sides of the circulating means, and the condensing temperature in the condensing means is adjusted to control the partial pressure of the water vapor of the anode off-gas, and the amount of water vapor circulating in the water vapor reformer is adjusted in combination with the circulating amount of the anode off-gas.
Since this control is essentially temperature control, a simple and highly reliable system can be constructed using readily available equipment.
The operation method of the fuel cell system having such a configuration is as follows:
a condensing means for removing condensed water on the discharge side, suction side, or both sides of the circulating means for circulating the anode off-gas to the steam reformer,
the circulation amount of the circulation means and the condensation temperature in the condensation means are adjusted to adjust the amount of steam circulated in the steam reformer.
The 3 rd feature of the present invention is constituted by:
in the above-described anode off-gas circulation path,
a cooling mechanism that cools the anode off-gas flowing through the anode off-gas circulation path is provided between the condensing mechanism and the fuel cell;
meanwhile, a temperature raising mechanism is provided between the condensing mechanism and the steam reformer, and the temperature raising mechanism raises the temperature of the anode off-gas flowing through the anode off-gas circulation path.
By virtue of the constitution of the present feature,
by providing the cooling means and the temperature increasing means with the condensing means interposed therebetween, the temperature of the gas flowing into the condensing means via the anode off-gas circulation path can be reduced by the cooling means, thereby reducing the load of the condensing means. On the other hand, the temperature of the gas discharged from the condensing means after the partial pressure of the steam is adjusted is reduced, and the temperature is required to be raised in the reforming reaction in the steam reformer, but the temperature can be raised by the temperature raising means to a temperature suitable for the input into the steam reformer.
The 4 th feature of the present invention is constituted by:
the temperature raising means uses heat recovered by the cooling means.
By adopting the present characteristic constitution, for example, the heat held by the anode off-gas discharged from the fuel cell operating at a higher temperature can be effectively used to control its water vapor partial pressure and the reforming reaction in the water vapor reformer. As a result, the energy efficiency in the entire fuel cell system improves.
The 5 th feature of the present invention is constituted by:
in the anode off-gas circulation path, the anode off-gas is cooled to above 50 ℃ and below 250 ℃.
By adopting the characteristic structure, the anode exhaust gas is temporarily cooled to the vicinity of the normal temperature and condensed water is removed, so that the volume flow rate of the fluid is reduced, and a common high-efficiency air pump of normal temperature specification can be used for recirculation, which is economical.
In addition, a compressor such as a pump is required to cool and circulate the anode off-gas, but the theoretical power efficiency of the compressor is improved by increasing the density of the gas due to the cooling, and on the other hand, an inexpensive compressor using an organic material or the like can be selected.
The 6 th feature of the present invention is constituted by:
wherein the composition is as follows: in an external power supply state in which generated power is supplied to the outside in accordance with the power load, only steam circulated through the anode off-gas circulation path flows into the steam reformer as necessary steam,
the condensation means and the control means function together as S/C ratio adjustment means for adjusting the S/C ratio, which is the ratio of the amount of water vapor circulated in the steam reformer to the amount of carbon fed into the steam reformer.
By employing this characteristic configuration, since the raw fuel and steam supplied to the steam reformer can be adjusted in such a manner that the supply ratio thereof is substantially dependent on the partial pressure of steam in the condensing means, the condensing means and the control means can be used as the adjusting means for the S/C ratio, which is the ratio of the amount of steam to the carbon to be supplied to the steam reformer and thus to the entire fuel cell system. As a result, by properly controlling the operation of the condensation mechanism when the load on the fuel cell fluctuates, the change and control can be arbitrarily performed, and the load tracking performance and reliability of the power generation can be improved.
The operation method of the fuel cell system having such a configuration is as follows:
wherein the composition is as follows: in an external power supply state in which generated power is supplied to the outside in accordance with the power load, only steam circulated through the anode off-gas circulation path flows into the steam reformer as necessary steam,
the condensing means is made to function as S/C ratio adjusting means for adjusting the S/C ratio, which is the ratio of the amount of steam circulated in the steam reformer to the amount of carbon fed into the steam reformer.
The 7 th feature of the present invention is constituted by:
wherein the composition is as follows: in an external power supply state in which generated power is supplied to the outside in accordance with the power load, only steam circulated through the anode off-gas circulation path flows into the steam reformer as necessary steam,
and, with respect to the above-mentioned electric load,
the ratio of the amount of steam to be circulated in the steam reformer to the amount of carbon to be fed to the steam reformer, that is, the S/C ratio, is set to an appropriate S/C ratio in advance,
meanwhile, as for the fuel gas concentration at the outlet of the fuel electrode, a minimum fuel gas concentration is set,
the condensing means and the control means cooperate to function as S/C ratio adjusting means for adjusting the S/C ratio to the appropriate S/C ratio,
meanwhile, the water vapor partial pressure of the anode off-gas circulated in the above-mentioned water vapor reformer is controlled to be: the hydrocarbon fuel may be fed to the steam reformer at a partial pressure of steam in an amount such that the fuel gas concentration at the fuel electrode outlet is maintained at or above the minimum fuel gas concentration.
By adopting the present characteristic configuration, the raw fuel and steam supplied to the steam reformer can be adjusted in such a manner that the supply ratio thereof is substantially dependent on the partial pressure of steam in the condensing means. The condensing means and the control means are used as means for adjusting the S/C ratio, which is the ratio of the amount of steam to the amount of carbon to be supplied to the steam reformer and thus to the entire fuel cell system, and the S/C ratio of the gas supplied to the steam reformer is adjusted to an appropriate S/C ratio. As a result, by properly controlling the operation of the condensation mechanism when the load on the fuel cell fluctuates, the change and control can be arbitrarily performed, and the load tracking performance and reliability of the power generation can be improved.
As described above, if the electric load increases, the fuel gas concentration at the fuel electrode outlet decreases and the water vapor concentration increases, but by controlling the amount of condensed water removed by the condensation mechanism, the state of hydrocarbon fuel and water vapor on the upstream side of the fuel cell and the water vapor reformer is appropriately controlled with respect to the increase in the amount of generated electricity (increase in water generated in the fuel cell), and the fuel gas concentration at the fuel electrode outlet is maintained at or above the minimum fuel gas concentration, whereby good operation can be maintained.
The operation method of the fuel cell system having such a configuration is as follows:
wherein the composition is as follows: in an external power supply state in which generated power is supplied to the outside in accordance with the power load, only steam circulated through the anode off-gas circulation path flows into the steam reformer as necessary steam,
and, for an electrical load,
the ratio of the amount of steam to the amount of carbon fed to the steam reformer, that is, the S/C ratio, is set in advance, and the minimum fuel gas concentration is set with respect to the fuel gas concentration at the outlet of the fuel electrode,
The S/C ratio is adjusted to the appropriate S/C ratio,
meanwhile, the water vapor partial pressure of the anode off-gas circulated in the above-mentioned water vapor reformer is controlled to be: the hydrocarbon fuel may be fed to the steam reformer at a partial pressure of steam in an amount such that the fuel gas concentration at the fuel electrode outlet is maintained at or above the minimum fuel gas concentration.
The 8 th feature of the present invention is constituted by:
there is a path for supplying at least a part of the above-described anode off-gas, the water vapor partial pressure of which has been reduced, to the desulfurization reactor.
By adopting the present characteristic constitution, the reducing gas contained in the gas can be used for desulfurization.
The operation method of the fuel cell system having such a configuration is as follows:
the desulfurization apparatus includes a path for supplying at least a part of the anode off-gas having a reduced water vapor partial pressure to the desulfurization reactor.
The 9 th feature of the present invention is constituted by:
the method comprises a desulfurization reactor for removing sulfur components supplied together with the hydrocarbon fuel, and supplying the hydrocarbon fuel desulfurized in the desulfurization reactor to a sulfur content of 1vol.ppb or less to the steam reformer.
By adopting the characteristic configuration, sulfur components (for example, odorizing agent added to city gas) supplied together with hydrocarbon fuel are removed, adverse effects of the sulfur components on the steam reformer, the fuel cell, and the like are reduced, and stable operation over a long period of time can be ensured.
It is further preferable that the hydrocarbon fuel desulfurized to a sulfur content of 0.1vol.ppb or less is supplied to the steam reformer, because adverse effects of sulfur components on the steam reformer, the fuel cell, and the like can be further reduced, and a more long-term stable operation can be ensured.
The operation method of the fuel cell system having such a configuration is as follows:
the desulfurization reactor is provided with a desulfurization reactor for removing sulfur components supplied together with the hydrocarbon fuel, and the desulfurization reactor is desulfurized to a sulfur content of preferably 1vol.ppb or less, more preferably 0.1vol.ppb or less, and then supplied to the steam reformer.
The 10 th feature of the present invention is constituted by:
the anode off-gas discharged from the fuel electrode is distributed and supplied as a steam reforming gas in the steam reformer and as a combustion gas used for heating the steam reforming.
With this feature, the anode off-gas also serves as a combustion fuel for the burner that is a heat source for the steam reforming reaction, and thus the fuel concentration of the anode off-gas is increased to stabilize combustion, thereby improving the reliability of the system.
The 11 th feature of the present invention is constituted by:
a condensing means for removing condensed water on the discharge side or suction side or both sides of the circulating means for circulating the anode off-gas to the steam reformer,
the control means adjusts the circulation amount of the circulation means and the condensing temperature in the condensing means to adjust the amount of steam circulated in the steam reformer,
meanwhile, it is constituted as follows: in an external power supply state in which generated power is supplied to the outside in accordance with the power load, only steam circulated through the anode off-gas circulation path flows into the steam reformer as necessary steam,
in addition, regarding the S/C ratio, which is the ratio of the amount of water vapor circulated in the steam reformer to the amount of carbon charged into the steam reformer, an appropriate S/C ratio corresponding to the electric power load is set,
the condensing means and the control means cooperate to function as S/C ratio adjusting means for adjusting the S/C ratio to the appropriate S/C ratio,
meanwhile, the water vapor partial pressure of the anode off-gas circulated in the above-mentioned water vapor reformer is controlled to be: the steam partial pressure of the hydrocarbon fuel may be added to the steam reformer in an amount equal to or greater than a total amount required for obtaining the generated electric power and a total amount of the hydrocarbon fuel required for maintaining the temperature of the steam reformer at a temperature required for steam reforming by using heat generated by combustion of a combustion component contained in at least a part of the anode off-gas.
By adopting the present characteristic configuration, the raw fuel and steam supplied to the steam reformer can be adjusted in such a manner that the supply ratio thereof is substantially dependent on the partial pressure of steam in the condensing means. The condensing means and the control means are used as means for adjusting the S/C ratio, which is the ratio of the amount of carbon to the steam reformer and thus to the entire fuel cell system, and the S/C ratio of the gas fed into the steam reformer is adjusted to an appropriate S/C ratio. As a result, by properly controlling the operation of the condensation mechanism when the load on the fuel cell fluctuates, the change and control can be arbitrarily performed, and the load tracking performance and reliability of the power generation can be improved.
Further, if the hydrocarbon fuel is to be used for generating heat for steam reforming while supplying the electric power load, it is necessary to set the amount of the hydrocarbon fuel to an amount commensurate with the total amount of both, but by controlling the amount of the condensed water removed by the condensation mechanism to be commensurate with the electric power load and the amount of fuel required for steam reforming, the state of the hydrocarbon fuel and steam on the upstream side of the fuel cell and the steam reformer is appropriately controlled, and a good operation state can be maintained.
The operation method of the fuel cell system having such a configuration is as follows:
wherein the composition is as follows: in an external power supply state in which generated power is supplied to the outside in accordance with the power load, only steam circulated through the anode off-gas circulation path flows into the steam reformer as necessary steam,
regarding the S/C ratio, which is the ratio of the amount of steam circulated in the steam reformer to the amount of carbon fed to the steam reformer, an appropriate S/C ratio corresponding to the electric power load is set,
the S/C ratio is adjusted to the appropriate S/C ratio,
meanwhile, the water vapor partial pressure of the anode off-gas circulated in the above-mentioned water vapor reformer is controlled to be: the steam partial pressure of the hydrocarbon fuel may be added to the steam reformer in an amount equal to or greater than a total amount required for obtaining the generated electric power and a total amount of the hydrocarbon fuel required for maintaining the temperature of the steam reformer at a temperature required for steam reforming by using heat generated by combustion of a combustion component contained in at least a part of the anode off-gas.
The 12 th feature of the present invention is constituted by:
the fuel cell is a solid oxide fuel cell.
According to the present characteristic configuration, the fuel gas reformed by the steam reformer can be directly supplied to the solid oxide fuel cell to generate electric power.
In addition, although the power generation operation temperature of the solid oxide fuel cell is in a high temperature range of 700 ℃ or higher, the heat in the temperature range is effectively utilized, and high-efficiency power generation can be realized.
Drawings
FIG. 1 is a view showing the structure of a fuel cell system according to the present invention;
fig. 2 is a diagram showing the constitution of a fuel cell system as a comparative example;
fig. 3 is a view showing another embodiment of the fuel cell system according to the present invention.
Detailed Description
Embodiments of the present invention will be described with reference to the accompanying drawings.
In the description, the structure of the fuel cell system S1 according to the present invention will be described, and the structure of the fuel cell system S2 as a comparative example will be described.
Fig. 1 shows a configuration of a fuel cell system S1 according to the present invention.
1. Constitution of fuel cell system
The fuel cell system S1 is a system constructed using a solid oxide fuel cell SOFC (hereinafter referred to as "fuel cell main body"), and is configured by including the following components: the fuel cell system includes a fuel cell main body 101, a fuel gas supply system 4 that supplies a fuel gas g1 (specifically, a gas containing hydrogen and carbon monoxide) to the fuel cell main body 101, an oxidizing gas supply system 5 that supplies an oxidizing gas g2 (specifically, air containing oxygen) to the fuel cell main body 101, and exhaust gas treatment systems 6 (6 a, 6 c) that treat exhaust gas (anode exhaust gas g3a, cathode exhaust gas g3 c) discharged from the fuel cell main body 101 at the time of power generation by the fuel cell main body 101.
The fuel cell main body 101 is connected to an electric load (for example, a power conditioner: not shown), and can take out generated power.
The fuel cell main body 101 is configured by including a plurality of fuel cell stacks s, and as shown in fig. 1, includes a fuel electrode (anode) s2 on one surface of a solid oxide solid electrolyte s1 and an air electrode (cathode) s3 on the other surface.
Here, examples of the constituent material of the solid oxide solid electrolyte s1 include: examples of the zirconia doped with at least one rare earth element selected from Y, sc of YSZ include: cerium oxide doped with at least one selected from rare earth elements, lanthanum gallate doped with at least one selected from Sr and Mg, or the like. In addition, composite materials of these materials can be produced.
As the catalyst layer of the fuel electrode s2, for example, a cermet of Ni and YSZ, in which the separator is an alloy or oxide containing Cr, laCrO may be used 3 Perovskite oxides such as those mentioned above, fe-Cr alloys as ferrite stainless steel, fe-Cr-Ni alloys as austenitic stainless steel, ni-Cr alloys as nickel-based alloys, and the like.
As the catalyst layer of the air electrode s3, for example, there may be employed: laMO is prepared 3 (La, AE) MO obtained by substituting part of La in (e.g., m= Mn, fe, co, ni) with alkaline earth metal AE (ae=sr, ca) 3 The perovskite type oxide of (2) may be an alloy or oxide containing Cr in the separator, and LaCrO may be used 3 Perovskite oxides such as those mentioned above, fe-Cr alloys as ferrite stainless steel, fe-Cr-Ni alloys as austenitic stainless steel, ni-Cr alloys as nickel-based alloys, and the like.
With this configuration, the fuel gas g1 is supplied to the fuel electrode s2 and the oxidizing gas g2 is supplied to the air electrode s3 during power generation.
The fuel gas supply system 4 receives a hydrocarbon fuel (for example, CH 4 City gas 13A, etc. as a main component) and steam v are supplied to a fuel electrode s2 of a plurality of fuel cell stacks s provided in the fuel cell main body 101, and the fuel gas g1 obtained by steam reforming by the steam reformer 1 is supplied to the steam reformer 1 serving as a core.
As is well known, the city gas 13A generally contains methane derived from natural gas as a main component, ethane, propane, and butane, and sulfur-containing substances (sulfur components) such as DMS (dimethyl sulfide) and TBM (t-butyl mercaptan) as an odorizing agent. The sulfur-containing material becomes a poisoning component for various devices (particularly, catalyst elements included in the devices) constituting the fuel cell system.
In this fuel cell system S1, a raw fuel pump P1 for supplying a raw fuel g0 is provided above the steam reformer 1, and a heater 2 for heating the supplied raw fuel g0 and a desulfurization reactor 3 for removing sulfur components contained in the raw fuel g0 from the raw fuel g0 are provided.
The desulfurization reactor 3 accommodates a copper-zinc-based desulfurizing agent, and the sulfur content of the sulfur component contained in the raw fuel g0 is reduced to 1vol.ppb or less (more preferably 0.1vol.ppb or less) in the desulfurization reactor 3. As such a copper-zinc-based desulfurizing agent, the following desulfurizing agents can be typically used: desulfurizing agents obtained by hydrogen-reducing a copper oxide-zinc oxide mixture prepared by a coprecipitation method using a copper compound (e.g., copper nitrate, copper acetate, etc.) and a zinc compound (e.g., zinc nitrate, zinc acetate, etc.); or a desulfurizing agent obtained by subjecting a copper oxide-zinc oxide-aluminum oxide mixture prepared by a coprecipitation method using a copper compound, a zinc compound, and an aluminum compound (for example, aluminum nitrate, sodium aluminate, or the like) to hydrogen reduction.
A burner 7 for heating the steam reformer 1 is provided.
A part of the anode off-gas g3a discharged from the fuel electrode s2 is supplied as a combustion gas to the burner 7, and the cathode off-gas g3c discharged from the air electrode s3 is supplied, so that the combustion components (hydrogen, hydrocarbon, and carbon monoxide) contained in the anode off-gas g3a are combusted by oxygen contained in the cathode off-gas g3 c. As a result, in this fuel cell system S1, the combustion component, which is the fuel leaked from the fuel electrode S2, is burned by the oxygen leaked from the air electrode S3 for steam reforming.
Therefore, the raw fuel g0 sucked and transported by the raw fuel pump P1 is desulfurized and then flows into the steam reformer 1. The remaining part of the anode off-gas g3a discharged from the fuel electrode s2 is circulated as a steam reforming gas by the circulation pump P2. In the present invention, this circulation path is referred to as an anode off-gas circulation path 8, and the circulation path 8 will be described later.
As described above, the steam reformer 1 is thermally connected to the burner 7, and the burner 7 burns the anode off-gas g3a discharged from the fuel cell main body 101 and steam reforms with heat generated in the burner 7.
The steam reformer 1 houses a steam reforming catalyst, and examples of such a catalyst include: ruthenium-based catalyst and nickel-based catalyst. Specifically, ru/Al obtained by supporting ruthenium component on alumina carrier can also be used 2 O 3 Catalyst or Ni/Al obtained by supporting nickel component on alumina carrier 2 O 3 A catalyst, etc.
In the case where the hydrocarbon fuel is methane, the reaction performed in the steam reformer 1 is an endothermic reaction performed by obtaining heat from the outside, as shown in the following 2 chemical formulas.
[ chemical formula 1]
CH 4 +H 2 O→CO+3H 2
[ chemical formula 2]
CH 4 +2H 2 O→CO 2 +4H 2
The oxidizing gas supply system 5 supplies the oxidizing gas g2 to the air electrode s3 of the plurality of fuel cell stacks s provided in the fuel cell main body 101.
In the illustrated fuel cell system S1, external air is sucked by the air blower B, and air is preheated in the heater 9 provided before the fuel cell main body 101 and then supplied to the air electrode S3. The source of air preheating may be, for example, exhaust gas discharged from the burner 7. That is, the heat recovery device 10ex is used to recover the heat of the exhaust gas gex of the combustor 7, and can be used for not only air preheating but also cooling to be sent to the steam reformer 1 specific to the present application and heating of the anode off gas g3a after the steam partial pressure is adjusted, and can be used for heat utilization such as hot water supply in general.
The exhaust gas treatment system 6 is two systems: an anode off-gas treatment system 6a that receives its off-gas from the fuel electrode s2 side of the fuel cell main body 101; and a cathode off-gas treatment system 6c receiving its off-gas from the air electrode s3 side.
The anode exhaust gas treatment system 6a is branched at the lower side by a distributor 10, one of which is connected to the burner 7 and the other to the steam reformer 1. In this distributor 10, the anode off-gas g3a is distributed so that the amount of anode off-gas supplied to the burner 7 is approximately 1, and the amount of anode off-gas supplied to the steam reformer 1 is approximately 3. The distribution ratio can be arbitrarily set according to the equipment conditions and the operation conditions of the system S1.
The heat recovery device 11, the cooler 12, the circulation pump P2, the humidifier 13, and the heater 14 are provided in this order from the distributor 10, when the description is given of the devices disposed in the circulation system of the anode off-gas g3a circulating in the steam reformer 1. Here, the heat recoverer 11 and the cooler 12 function as cooling means for cooling the gas g3a flowing through the inside in a gas phase or a mixed phase in a stepwise manner, and the condensed water W1 is discharged from the cooler 12, and the condensed water W1 is generated by cooling mainly determined by the heat-resistant temperature of the pump. The humidifier 13 is a so-called condenser, and functions as a condensing means for condensing and removing the water vapor v contained in the gas g3a flowing through the inside to discharge the condensed water W2. In this humidifier 13, therefore, the water vapor partial pressure of the gas flowing through the inside is adjusted according to the condensation temperature thereof. The condenser 12 is provided with a function of adjusting the condensation temperature in the humidifier 13 according to the plant conditions and the operation conditions, so that the humidifier 13 can be omitted. In this configuration, the cooler 12 functions as a condensing mechanism in the present invention. The heater 14 functions as a temperature increasing means for directly increasing the temperature of the gas g3a flowing through the inside in the gas phase. The operating conditions of these devices 11, 12, P2, 13, 14 will be described in detail with respect to the temperature and the like of the gas g3a flowing through each portion in terms of the operating conditions of the fuel cell system S1 described later.
A part of the anode off-gas g3a, the water vapor partial pressure of which has been adjusted, is circulated through the mixer 15 by the circulation pump P2, is used for water vapor reforming in the water vapor reformer 1, and is supplied to the fuel electrode s2 of the fuel cell main body 101.
Therefore, in this fuel cell system S1, in an external power supply state in which generated power is supplied to the outside in accordance with the power load, the steam v for steam reforming in the steam reformer 1 is supplied through the anode off-gas circulation path 8 (anode off-gas treatment system 6 a).
In fig. 1, a control element constituting an essential part of the present invention is shown with respect to a control part C of the fuel cell system S1. The control unit C receives the electric load required by the system S1 and outputs an instruction to the raw fuel pump P1 in accordance with the raw fuel supply amount required to correspond to the electric load. Then, the corresponding air supply amount is instructed to the blower B.
Further, in order to maintain the S/C ratio in the steam reformer 1 in a desired state, the anode off-gas circulation amount is indicated to the circulation pump P2, and at the same time, the humidifier outlet temperature is indicated to the humidifier 13. As a result, the partial pressure of the anode off-gas g3a mixed with the raw fuel g0 is maintained in an appropriate range by returning the anode off-gas circulation path 8 to the mixer 15, and the ratio of the amount of steam to the amount of carbon (molar ratio) that is the ratio of the amount of steam to the amount of carbon (S/C ratio) that is the ratio of the amount of steam that is fed to the fuel cell system S1 (specifically, the steam reformer 1) is maintained in a desired state, whereby highly efficient power generation can be performed.
Therefore, the control unit C becomes a control mechanism in the present invention. The control means and the condensing means of the present invention cooperate to form an S/C ratio adjusting means of the fuel cell system S1.
Next, a fuel cell system S2, which is a comparative example to the present invention, will be described with reference to fig. 2. In this comparative example, the same reference numerals are given to the same devices as those shown in fig. 1.
The fuel cell system S2 is also configured with a conventional solid oxide fuel cell SOFC as a core, but when power generation is performed with a hydrocarbon as the raw fuel g0, the steam v contained in the combustion gas of the hydrocarbon fuel generated in the steam reformer 1 is condensed by the condenser 22, and the steam v obtained by evaporating the condensed water w is used for steam reforming. In this system configuration, since water contained in the combustion gas is used for steam reforming, the water quality maintenance device 20 is required.
There is no change in the provision of a fuel cell (fuel cell main body 101) in which the reformed gas obtained by the steam reformer 1 is supplied to the fuel electrode s2 as the fuel gas g1 and the oxidizing gas g2 is supplied separately to perform the cell reaction.
As is also apparent from the same drawing, the oxidizing gas supply system 5 for supplying the oxidizing gas g2 to the air electrode s3 is provided with a blower B and a heater 9, and the cathode off-gas g3c discharged from the air electrode s3 is introduced into the steam reformer 1 for combustion in the heating burner V provided in the apparatus. At a portion surrounded by the central tank of the steam reformer 1, a reforming reaction portion 1V for performing steam reforming using the steam V generated by the evaporation portion 23 is provided.
The description will be made of the supply of the raw fuel g0 and the supply of the steam v to the steam reformer 1, and the supply of the raw fuel g0 is performed by desulfurizing the raw fuel in the desulfurization reactor 3 after the temperature raising operation by the heater 2 in this example, and the reforming in the steam reformer 1 is performed. On the other hand, as for the supply of the steam V, the anode off-gas g3a is burned using the burner V provided in the steam reformer 1, and water contained in the burned gas is recovered by heat recovery (performed by the heat recovery device 21) and condensed (performed by the condenser 22). The condensed water w is subjected to water quality treatment by the water quality maintaining device 20 and then supplied to the steam reformer 1. The steam v required for reforming can be obtained by the evaporation unit 23 provided in the steam reformer 1.
In this way, the fuel gas g1 subjected to steam reforming is supplied to the fuel electrode s2 for power generation. Here, the anode off-gas g3a discharged from the fuel electrode s2 contains fuel as a combustion component, and is supplied to the burner V provided in the steam reformer 1 for combustion.
Therefore, the configuration of this comparative example does not include the anode off-gas circulation path 8 described in the present invention described in the foregoing embodiment, and of course does not include the distributor 10, the heat recovery unit 11, the cooler 12, the circulation pump P2, the humidifier 13, and the heater 14.
[ study results ]
The present invention will be described below with reference to the results of studies conducted by the inventors.
Operating conditions of the fuel cell system S1
1. The anode off-gas g3a (temperature 700 ℃) discharged from the fuel electrode s2 of the solid oxide fuel cell SOFC as the fuel cell main body 101 is distributed to the circulation amount on the steam reformer 1 side using the distributor 10: input to burner 7 side = 3:1.
2. the anode off-gas g3a is cooled to about 320 c by the heat recoverer 11 and then further cooled to about 85 c by the cooler 12. The fluid on the heated side of the heat recovery 11 may be the gas flowing through the heaters 2, 9, 14, etc. described earlier. That is, the heat recovered in the cooling of the anode off-gas g3a can be used for reheating (warming up) the anode off-gas g3a, preheating the oxidizing gas g2, and heating the raw fuel g 0.
Since the excessive condensed water W1 is generated in the cooler 12, it becomes an obstacle to driving the air pump, and is discharged.
3. The anode off-gas g3a cooled to 85 ℃ was pressurized to 20kPa by the circulation pump P2, while the temperature was raised to 98.3 ℃ by the heat-insulating compression accompanying the pressurization. The theoretical power of the circulation pump P2 in this case is only 0.66% of the dc output of the fuel cell described later.
4. The anode off-gas g3a compressed by the circulation pump P2 is cooled again to 88.5 ℃ by the humidifier 13 while discharging the remaining condensed water w2. The condensing temperature here determines (adjusts) the water vapor partial pressure of the anode off-gas g3 a.
5. The anode off-gas g3a, the water vapor partial pressure of which has been adjusted, is heated to 300 ℃ in the heater 14 by the heat recovered by the heat recovery device 11, and is supplied to the water vapor reformer 1 together with the desulfurized raw fuel g 0.
When the fuel cell system S1 is operated under the above-described operation conditions, the reaction equilibrium temperature of the steam reformer 1 can be maintained at 670 ℃.
Then, when the power generation voltage of the fuel cell was set to 0.8V at the time of normal operation and the fuel utilization ratio ([ amount of fuel consumed by the fuel cell ]/[ amount of fuel supplied to the fuel cell ]) was set to 68.8%, the ac output efficiency of the cell ([ ac output of the fuel cell ]/[ fuel energy (enthalpy) per unit time of the city gas (13A) supplied to the fuel cell system as the raw fuel) ] was set to 61.2%, and the dc output efficiency ([ dc output of the fuel cell ]/[ fuel energy (enthalpy) per unit time of the city gas (13A) supplied to the fuel cell system as the raw fuel) was set to 68.3% based on LHV standard.
In this operation example, the concentration (corresponding to the partial pressure) of the raw fuel at the inlet and outlet of the fuel electrode was about 0.3 vol%, and the concentration (corresponding to the partial pressure) of the fuel gas (hydrogen+carbon monoxide) at the inlet and outlet of the fuel electrode was about 49 vol% and about 15 vol%. The water vapor concentration (corresponding to the partial pressure) at the inlet and outlet of the fuel electrode was 26vol.% and 50vol.%.
Then, about 7.45% of the steam circulated to the steam reformer 1 via the anode off-gas circulation path 8 is removed by the condensation mechanisms 12 and 13.
Hereinafter, a fuel cell system S2 as a comparative example will be described. In the study, the standard was a standard of city gas (13A) as a raw fuel and the current latest solid oxide fuel cell SOFC was combined, and the parameters (dc/ac conversion efficiency, etc.) required for the study were the same except for the present invention and were strictly compared.
In the fuel cell system S2 of the comparative example, when the S/C ratio in the steam reformer 1 is maintained at 2.6, the anode off-gas g3a can be burned by using the cathode off-gas g3C in the burner V provided in the steam reformer 1 to maintain the reaction equilibrium temperature at 670 ℃.
However, in this condition, when the power generation voltage of the fuel cell was set to 0.8V at the time of normal operation and the fuel utilization was set to 80.0%, the ac output efficiency of the cell was 52.1%, and the dc output efficiency was 57.8% based on the LHV standard.
In this comparative example, the concentration (corresponding to the partial pressure) of the raw fuel at the inlet and outlet of the fuel electrode was 1vol.%, and the concentration (corresponding to the partial pressure) of the fuel gas (hydrogen+carbon monoxide) at the inlet and outlet of the fuel electrode was 67vol.%, and 14vol.%. The water vapor concentration (corresponding to the partial pressure) at the inlet and outlet of the fuel electrode was 23vol.% and 70vol.%. In this comparative example, the anode off-gas is discharged to the outside only after being used for heating of the steam reformer 1.
In this example, if the same power generation efficiency as in the present invention is desired, the fuel utilization of the fuel cell needs to be raised to 80.0% (fuel utilization) × (68.3% (dc output efficiency in the present invention)/(57.8% (dc output efficiency in the comparative example))= 94.53%. In a general industrial fuel cell system, this value is far more than 85 to 90% of the critical value allowable by fuel distribution uniformity and control error, and it is extremely difficult to realize practical use.
In this situation, in the present invention, as described above, the anode off-gas is circulated to the steam reformer 1, and the fuel utilization rate is reduced and the fuel partial pressure (fuel gas partial pressure) at the fuel cell outlet is ensured at the same time, whereby the power generation amount (power generation efficiency) can be improved.
That is, the fuel utilization rate of the fuel cell can be reduced by increasing the fuel supply amount to the fuel cell by the circulation of the anode off-gas and the adjustment (humidity control) of the water vapor partial pressure at the time of introducing the water vapor reformer 1 (by relaxing the fuel consumption rate required for the fuel cell, it is advantageous for the performance of the fuel cell, and also advantageous for the durability by making the partial pressure difference at the inlet and outlet of the fuel electrode small, and by averaging the temperature distribution accompanying the power generation. On the other hand, regarding the S/C ratio, in order to cool the gas temperature using a general-purpose pump, it is possible to adjust to a target value (2.6 in this case) close to a certain degree. The fuel cell system according to the present invention can be operated in the S/C ratio of 1.5 to 3.5, preferably 1.5 to 3.0, for example, but since the sulfur content of the hydrocarbon fuel fed to the steam reformer 1 has been reduced to 1vol.ppb, stable operation can be achieved over a long period of time even under conditions in which the S/C ratio is relatively low.
Therefore, in the present embodiment, the humidity controller 13 functioning as the condensing means works together with the control unit C functioning as the control means, and functions as S/C ratio adjusting means for adjusting the S/C ratio, which is the ratio of the amount of steam circulated in the steam reformer 1 to the amount of carbon fed into the steam reformer 1.
In this case, the amount of water vapor (corresponding to the water vapor partial pressure) of the raw fuel that can be introduced into the steam reformer 1 can be determined by determining the fuel gas flow rate at the fuel electrode inlet from the minimum fuel gas flow rate at the fuel electrode outlet (corresponding to the minimum fuel gas concentration of the present invention) and determining the amount of hydrocarbon fuel that is the raw fuel introduced into the steam reformer 1 from the S/C ratio set in the steam reformer 1 based on the fuel utilization rate in the fuel cell.
The target value of the S/C ratio is a value set in advance as an appropriate S/C ratio corresponding to the electric power load, and the minimum fuel gas concentration (for example, 12 vol.%) is set as the minimum limit value for the fuel gas concentration at the fuel electrode outlet.
As a result, in the present embodiment, in the external power supply state in which generated power is supplied to the outside in accordance with the power load, only the steam circulated through the anode off-gas circulation path 8 flows into the steam reformer 1 as the necessary steam v, and the condensation means and the control means act together as S/C ratio adjustment means for adjusting the S/C ratio to an appropriate S/C ratio, and in this operation mode,
(1) Regarding the fuel gas concentration at the outlet of the fuel electrode s2, the water vapor partial pressure of the anode off-gas g3a circulated in the water vapor reformer 1 is controlled to be: hydrocarbon fuel in an amount such that the fuel gas concentration at the outlet of the fuel electrode can be maintained at or above the minimum fuel gas concentration can be fed to the partial pressure of water vapor in the water vapor reformer 1;
(2) Regarding the generation of electricity commensurate with the electric power load and the generation of heat required for steam reforming, the steam partial pressure of the anode off-gas g3a circulating in the steam reformer 1 is controlled as: the steam partial pressure of the hydrocarbon fuel may be fed to the steam reformer 1 in an amount equal to or greater than the total amount of the hydrocarbon fuel required to obtain the generated electric power and the total amount of the hydrocarbon fuel required to maintain the temperature of the steam reformer 1 at the temperature required for steam reforming by using the heat generated by the combustion of the combustion component contained in at least a part of the anode off-gas.
Other embodiments
(1) In the above embodiment, the configuration in which the heat recovery device 11, the cooler 12, the circulation pump P2 as the circulation means, the humidity regulator 13 as the condensation means, and the heater 14 are provided in the anode off-gas circulation path 8 is shown. In the present invention, the manner of temperature increase after the cooling of the anode off-gas and the partial pressure adjustment of the water vapor (after removal of the condensed water) is not limited as long as the amount of water vapor to be circulated in the water vapor reformer 1 can be appropriately adjusted via the anode off-gas circulation path 8. For example, the anode off-gas may be cooled to a condensable state by a single mechanism without separately providing the heat recoverer 11 and the cooler 12. Therefore, the mechanism that cools the anode off-gas before adjusting the water vapor partial pressure is referred to as a cooling mechanism.
On the other hand, after the steam partial pressure is adjusted, the anode off-gas may be heated to a state suitable for steam reforming in the steam reformer, and this means is referred to as a temperature-increasing means.
Here, although the functions of the cooler 12 and the humidifier 13 are described, as described in the embodiment shown above, the condensation temperature adjusting function is added to the cooler 12 according to the plant conditions and the operation conditions, and thus the humidifier 13 may be omitted. In the case where the cooling means and the condensing means are omitted in this way, the cooler 12 functions as the cooling means and the condensing means in the present invention.
The position of the condensing means in the anode off-gas circulation path 8 may be either the discharge side (the condenser 13 in the previous embodiment) or the suction side (the cooler 12 in the previous embodiment) of the circulation means, or both. Of course, either or both of the cooler 12 and the humidifier 13 may be provided.
(2) In the above-described embodiment, the description has been mainly made of the state in which the fuel cell system supplies the generated power to the outside (the external power supply state) while keeping track of the power load.
As described above, in such a normal operation, the fuel cell system S1 can generate electricity satisfactorily only by using the steam contained in the anode off-gas circulated through the anode off-gas circulation path 8 in the steam reformer 1, and can be configured as follows, for example: at the time of starting the fuel cell system S1, before electric power is supplied to the outside, the steam reformer 1 is supplied with steam v in addition to the raw fuel via the raw fuel supply system to supply steam necessary at the time of starting the cell to the steam reformer 1.
(3) In the above embodiment, the anode off-gas is cooled to 85 ℃ in the anode off-gas circulation path 8 as the operation condition thereof, but is preferably cooled to more than 50 ℃ and less than 250 ℃ from the viewpoint of using general-purpose products for the circulation pump P2 and the humidifier 13.
(4) In the above embodiment, the steam reforming reaction has been mainly described, but the reforming reaction may be a combination of the steam reforming reaction and the partial combustion reforming reaction, or a combination of the steam reforming reaction and the carbon dioxide reforming (dry reforming).
(5) In the above embodiment, the steam supplied to the steam reformer 1 is the steam v introduced into the mixer 15 through the heater 14. However, as shown in fig. 3, at least a part of the anode off-gas g3a discharged from the humidifier 13, the water vapor partial pressure of which has been reduced, may be introduced to the upstream side of the heater 2 via the branch line 8a and mixed with the raw fuel, while being heated by the heater 2 and introduced into the desulfurization reactor 3. By doing so, the anode off-gas g3a having a reduced water vapor partial pressure can be used for desulfurization.
Symbol description
1: a steam reformer;
8: an anode off-gas circulation path;
11: a heat recoverer (cooling mechanism);
12: a cooler (cooling mechanism/condensing mechanism);
13: a humidity controller (condensing mechanism);
14: a heater (temperature raising mechanism);
101: a fuel cell main body (solid oxide fuel cell SOFC);
g0: raw fuel (CH) 4 ：13A)；
g1: fuel gas (H) 2 、CO)；
g2: oxidizing gas (air);
g3a: anode off-gas;
g3c: cathode exhaust gas;
s: a fuel cell stack;
s1: solid oxide solid electrolyte;
s2: a fuel electrode (anode);
s3: an air electrode (cathode);
c: a control unit (control mechanism).

Claims (15)

1. A fuel cell system is provided with:
a steam reformer for reforming a hydrocarbon fuel through a steam reforming reaction;
a fuel cell that functions by introducing the reformed gas obtained by the steam reformer to a fuel electrode; and
an anode off-gas circulation path for cooling the anode off-gas discharged from the fuel electrode, removing condensed water, and introducing the cooled water into the steam reformer,
wherein the fuel cell system includes a control means for controlling the partial pressure of water vapor of the anode off-gas circulated in the steam reformer by adjusting the amount of condensed water removed from the anode off-gas circulation path,
A condensing means for removing condensed water on the discharge side or suction side or both sides of the circulating means for circulating the anode off-gas to the steam reformer,
the control means is configured to adjust the circulation amount of the circulation means and the condensation temperature in the condensation means to adjust the amount of steam circulated in the steam reformer,
comprises a mixer for mixing the hydrocarbon fuel with the anode off-gas from which condensed water has been removed by the condensing means, and the mixed gas is fed from the mixer to the steam reformer,
the fuel cell system is constituted as follows: in an external power supply state in which generated power is supplied to the outside in accordance with the power load, only steam circulated through the anode off-gas circulation path flows into the steam reformer as necessary steam,
the ratio of the amount of steam to be circulated in the steam reformer to the amount of carbon to be fed to the steam reformer, that is, the S/C ratio, is set to an appropriate S/C ratio in advance,
the condensing means and the control means function together as S/C ratio adjusting means for adjusting the S/C ratio, which is the ratio of the amount of water vapor circulated in the steam reformer to the amount of carbon fed into the steam reformer.
2. The fuel cell system according to claim 1, wherein, in the anode off-gas circulation path,
a cooling mechanism that cools the anode off-gas flowing through the anode off-gas circulation path is provided between the condensing mechanism and the fuel cell;
meanwhile, a temperature raising mechanism is provided between the condensing mechanism and the steam reformer, and the temperature raising mechanism raises the temperature of the anode off-gas flowing through the anode off-gas circulation path.
3. The fuel cell system according to claim 2, wherein the temperature raising means uses heat recovered by the cooling means.
4. A fuel cell system according to any one of claims 1 to 3, wherein in the anode off-gas circulation path, the anode off-gas is cooled to a temperature higher than 50 ℃ and lower than 250 ℃.
5. A fuel cell system is provided with:
a steam reformer for reforming a hydrocarbon fuel through a steam reforming reaction;
a fuel cell that functions by introducing the reformed gas obtained by the steam reformer to a fuel electrode; and
an anode off-gas circulation path for cooling the anode off-gas discharged from the fuel electrode, removing condensed water, and introducing the cooled water into the steam reformer,
Wherein the fuel cell system includes a control means for controlling the partial pressure of water vapor of the anode off-gas circulated in the steam reformer by adjusting the amount of condensed water removed from the anode off-gas circulation path,
a condensing means for removing condensed water on the discharge side or suction side or both sides of the circulating means for circulating the anode off-gas to the steam reformer,
the control means is configured to adjust the circulation amount of the circulation means and the condensation temperature in the condensation means to adjust the amount of steam circulated in the steam reformer,
the fuel cell system is constituted as follows: in an external power supply state in which generated power is supplied to the outside in accordance with the power load, only steam circulated through the anode off-gas circulation path flows into the steam reformer as necessary steam,
with respect to the above-mentioned electrical load,
the ratio of the amount of steam to be circulated in the steam reformer to the amount of carbon to be fed to the steam reformer, that is, the S/C ratio, is set to an appropriate S/C ratio in advance,
meanwhile, as for the fuel gas concentration at the outlet of the fuel electrode, a minimum fuel gas concentration is set,
The condensing means and the control means cooperate to function as S/C ratio adjusting means for adjusting the S/C ratio to the appropriate S/C ratio,
meanwhile, the water vapor partial pressure of the anode off-gas circulated in the above-mentioned water vapor reformer is controlled to be: the hydrocarbon fuel may be fed to the steam reformer at a partial pressure of steam in an amount such that the fuel gas concentration at the fuel electrode outlet is maintained at or above the minimum fuel gas concentration.
6. The fuel cell system according to claim 1 or 5, comprising a desulfurization reactor for removing sulfur components supplied together with the hydrocarbon fuel, and a path for supplying at least a part of the anode off-gas having a reduced water vapor partial pressure to the desulfurization reactor.
7. The fuel cell system according to claim 1 or 5, comprising a desulfurization reactor for removing sulfur components supplied together with the hydrocarbon fuel, wherein the hydrocarbon fuel desulfurized in the desulfurization reactor to a sulfur content of 1vol.ppb or less is supplied to the steam reformer.
8. The fuel cell system according to claim 1 or 5, wherein the anode off-gas discharged from the fuel electrode is distributed and supplied as a steam reforming gas in the steam reformer and as a combustion gas used for heating steam reforming.
9. The fuel cell system according to claim 8, wherein the partial pressure of water vapor of the anode off-gas circulated in the water vapor reformer is controlled to be: the steam partial pressure of the hydrocarbon fuel may be added to the steam reformer in an amount equal to or greater than a total amount required for obtaining the generated electric power and a total amount of the hydrocarbon fuel required for maintaining the temperature of the steam reformer at a temperature required for steam reforming by using heat generated by combustion of a combustion component contained in at least a part of the anode off-gas.
10. The fuel cell system according to claim 1 or 5, wherein the fuel cell is a solid oxide fuel cell.
11. A method for operating a fuel cell system, the fuel cell system comprising:
a steam reformer for reforming a hydrocarbon fuel through a steam reforming reaction;
a fuel cell that functions by introducing the reformed gas obtained by the steam reformer to a fuel electrode; and
an anode off-gas circulation path for cooling the anode off-gas discharged from the fuel electrode, removing condensed water, and introducing the cooled water into the steam reformer,
Wherein the amount of condensed water removed from the anode off-gas circulation path is adjusted to adjust the water vapor partial pressure of the anode off-gas circulated in the water vapor reformer,
the fuel cell system is constituted as follows: in an external power supply state in which generated power is supplied to the outside in accordance with the power load, only steam circulated through the anode off-gas circulation path flows into the steam reformer as necessary steam,
in the case of an electrical load,
the ratio of the amount of steam to the amount of carbon fed to the steam reformer, that is, the S/C ratio, is set in advance, and the minimum fuel gas concentration is set with respect to the fuel gas concentration at the outlet of the fuel electrode,
the S/C ratio is adjusted to the appropriate S/C ratio,
meanwhile, the water vapor partial pressure of the anode off-gas circulated in the above-mentioned water vapor reformer is controlled to be: the hydrocarbon fuel may be fed to the steam reformer at a partial pressure of steam in an amount such that the fuel gas concentration at the fuel electrode outlet is maintained at or above the minimum fuel gas concentration.
12. A method for operating a fuel cell system, the fuel cell system comprising:
A steam reformer for reforming a hydrocarbon fuel through a steam reforming reaction;
a fuel cell that functions by introducing the reformed gas obtained by the steam reformer to a fuel electrode; and
an anode off-gas circulation path for cooling the anode off-gas discharged from the fuel electrode, removing condensed water, and introducing the cooled water into the steam reformer,
wherein the amount of condensed water removed from the anode off-gas circulation path is adjusted to adjust the water vapor partial pressure of the anode off-gas circulated in the water vapor reformer,
the composition is as follows: in an external power supply state in which generated power is supplied to the outside in accordance with the power load, only steam circulated through the anode off-gas circulation path flows into the steam reformer as necessary steam,
regarding the S/C ratio, which is the ratio of the amount of steam circulated in the steam reformer to the amount of carbon fed to the steam reformer, an appropriate S/C ratio corresponding to the electric power load is set,
the S/C ratio is adjusted to the appropriate S/C ratio,
meanwhile, the water vapor partial pressure of the anode off-gas circulated in the above-mentioned water vapor reformer is controlled to be: the steam partial pressure of the hydrocarbon fuel may be added to the steam reformer in an amount equal to or greater than a total amount required for obtaining the generated electric power and a total amount of the hydrocarbon fuel required for maintaining the temperature of the steam reformer at a temperature required for steam reforming by using heat generated by combustion of a combustion component contained in at least a part of the anode off-gas.
13. The method for operating a fuel cell system according to claim 11 or 12, wherein the fuel cell system comprises a condensing means for removing condensed water on a discharge side or a suction side or both sides of a circulating means for circulating the anode off-gas to the steam reformer,
the circulation amount of the circulation mechanism and the condensing temperature in the condensing mechanism are adjusted to adjust the amount of water vapor circulated in the above-mentioned steam reformer.
14. The method for operating a fuel cell system according to claim 11 or 12, wherein the fuel cell system comprises a desulfurization reactor for removing sulfur components supplied together with the hydrocarbon fuel, and at least a part of the anode off-gas having a reduced water vapor partial pressure is supplied to the desulfurization reactor.
15. The method for operating a fuel cell system according to claim 14, wherein a desulfurization reactor for removing sulfur components supplied together with the hydrocarbon fuel is provided, and the hydrocarbon fuel desulfurized in the desulfurization reactor to a sulfur content of 1vol.ppb or less is supplied to the steam reformer.
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